Oil-shale, a sedimentary rock found in vast formations in several parts of the world, contains about 5 to about 60 wt. % organic material. About 10% of this organic material is extractable with organic solvents and is called bitumen. The remainder of the organic material is insoluble in organic and inorganic solvents and is a crosslinked macromolecular material called kerogen. The remainder of the oil-shale is inorganic and is composed of various minerals including clays. Much work has recently been done to develop efficient methods for acquiring useful liquids and gases from oil-shale as replacements for petroleum products. The inherent mineral matter in oil-shale represents significant debits in both the handling and processing of the resource. Typically, the organic material which can be converted to useful fuels and chemical feedstocks, preferably liquids, represents only about 10-25 wt. % of the oil-shale. Furthermore, during processing there is considerable increased abrasive wear on preparation and conversion plants owing to the nature of the mineral matter contained in oil-shale. Also, relatively large reactor sizes are required to accommodate the higher material throughputs necessitated by the large quantities of mineral matter. Most important, is the fact that large quantities of rock must be heated to relatively high temperatures in order to break-down the kerogen of the oil-shale to liquid and gaseous products. This results in a loss in energy efficiency because much of this heat is not recoverable as useful work. In processes which include a catalyst during processing, there is a significant potential that the mineral matter, inherently present in the oil-shale, will contribute to catalyst poisoning. Consequently, various processes have been developed over the years to beneficiate, or demineralize, oil-shale prior to further processing to concentrate the organic material and to alleviate the above-mentioned problems associated with oil-shale processing.
Although both physical and chemical methods have been developed for separating organic material from oil-shale thereby leaving substantially all of the mineral matter behind, chemical methods usually are superior over physical methods because of better separations obtained. One common type of chemical method for the separation of kerogen from oil-shale is acid dissolution of the inorganics. Such a technique typically involves the digestion of oil-shale with hydrochloric acid, hydrofluoric acid, or both. The hydrochloric acid dissolves the carbonate minerals and the hydrofluoric acid dissolves the silicate and clay minerals, thereby leaving a kerogen material containing about 5 to 10 wt. % mineral matter. Although such schemes have met with varying degrees of success, they suffer the disadvantage of being corrossive and, in most cases the inability to regenerate the acid reagents economically for recycle makes the process unattractive.
A variation in this acid technique is disclosed by R. D. Thomas in Fuel, 48, 75-80 (1969) wherein all of the mineral carbonates are removed from Green River oil-shale by treatment with carbonic acid. Thomas discloses that after sodium hydroxide washing to remove fine clay-like particles, the carbonate-free material can be centrifuged in a saturated aqueous calcium chloride solution to yield a concentrate containing 85-90 wt. % of the original kerogen with an ash content of less than 30 wt. %. Attempts to reproduce the physical separation results after demineralization with carbonic acid have been unsuccessful. See Vandergrift, G. F., Winans, R. E., and Horowitz, E. P., Fuel, 59, 634-6 (1980).
A variation of using acids to enrich the kerogen content of oil-shales which are rich in carbonates involves leaching by use of a sulfuric acid medium generated by the oxidizing capabilities of specific bacteria which convert pyrite in the oilshale to sulfuric acid. The sulfuric acid produced decomposes the carbonate minerals in the oil-shale leaving a kerogen enriched material containing significant amounts of silica. See U.S. Pat. No. 4,043,884.
Other approaches to chemical beneficiation include extraction of low grade Western U.S. oil-shales with aliphatic amides to recover 69 wt. % to 83 wt. % of the organics (U.S. Pat. No. 4,357,230), and treatment of various oil-shales with a 1:1 mixture of trichloroisocyanuric acid and a chlorine donor such as calcium hypochlorite in the presence of a petroleum based naphtha solvent giving recoveries of organics up to about 98 wt. % (U.K. Pat. No. 2,090,858).
Furthermore, disclosed in Intl. Patent Appl. Wo 82/03404 is a two-step treatment of oil-shale with 16 parts of fused alkali metal caustic at 350.degree. C. for 40 minutes followed by a sulfuric acid wash which led to recovery of 86 wt. % of the organics. This type of treatment suffers from the disadvantage that it may have a detrimental effect on the kerogen structure and reactivity.
Although various methods have been used, including physical grinding combined with chemical treatment, for beneficiating oil-shale, such prior art methods have not been able to remove a significant amount of mineral matter from the organic material under relatively mild conditions without the reagent being simultaneously and irreversibly destroyed. Consequently, there is still a need in the art for processes which are able to beneficiate oil-shale under relatively mild conditions and leave the beneficiation reagent in a form which is easily regenerable. Furthermore, none of the prior art methods suggest the use of ammonium salts for beneficiating oil-shales.